Catalyst and method for reduction of nitrogen oxides

ABSTRACT

A Selective Catalytic Reduction (SCR) catalyst was prepared by slurry coating ZSM-5 zeolite onto a cordierite monolith, then subliming an iron salt onto the zeolite, calcining the monolith, and then dipping the monolith either into an aqueous solution of manganese nitrate and cerium nitrate and then calcining, or by similar treatment with separate solutions of manganese nitrate and cerium nitrate. The supported catalyst containing iron, manganese, and cerium showed 80 percent conversion at 113 degrees Celsius of a feed gas containing nitrogen oxides having 4 parts NO to one part NO 2 , about one equivalent ammonia, and excess oxygen; conversion improved to 94 percent at 147 degrees Celsius. N 2 O was not detected (detection limit: 0.6 percent N 2 O).

RELATED CASES

This application is a continuation in part of U.S. patent applicationSer. No. 10/899,749 entitled “Catalyst and Method for Reduction ofNitrogen Oxides,” filed Jul. 27, 2004, now U.S. Pat. No. 7,378,069hereby incorporated by reference. The application also claims thebenefit of U.S. patent application Ser. No. 60/663,065 entitled“Catalyst and Method for Reduction of Nitrogen Oxides,” filed Mar. 18,2005, hereby incorporated by reference.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.W-7405-ENG-36 awarded by the U.S. Department of Energy. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to the abatement of nitrogenoxides and more particularly to the Selective Catalytic Reduction (SCR)of nitrogen oxides using a zeolite catalyst impregnated with iron,cerium, and manganese.

BACKGROUND OF THE INVENTION

It is generally recognized that nitric oxide (NO), nitrogen dioxide(NO₂), and nitrous oxide (N₂O) are principle contributors to smog andother undesirable environmental effects when they are discharged intothe atmosphere. NO_(x) is the term generally used to represent nitricoxide (NO), nitrogen dioxide (NO₂), and nitrous oxide (N₂O), as well asmixtures containing these gases. NO_(x) forms in the high temperaturezones of combustion processes. The internal combustion engine, and coalor gas-fired or oil-fired furnaces, boilers and incinerators, allcontribute to NO_(x) emissions. Although the concentrations of NO_(x) inthe exhaust gases produced by combustion usually are low, the aggregateamount discharged in industrial and/or highly populated areas tends tocause problems. NO_(x) is also produced during a variety of chemicalprocesses such as the manufacture of nitric acid, the nitration oforganic chemicals, the production of adipic acid, and the reprocessingof spent nuclear fuel rods.

In general, fuel-rich combustion mixtures produce exhaust gases withless NO_(x) than do lean fuel-air mixtures, i.e. mixtures in which moreair is provided than the stoichiometric amount required to completelycombust the fuel. Lean fuel mixtures will produce an exhaust gas thatcontains gaseous oxygen.

The US Environmental Protection Agency is requiring greater levels ofNO_(x) abatement from mobile and stationary emission sources. For ‘lightduty’ mobile sources such as light trucks and cars, NO_(x) emissionswill be required to not exceed 0.07 grams/mile, down from the currentlevel of around 0.8 grams/mile. This represents a NO_(x) abatementrequirement of greater than 90% over current technology. Some of thisabatement will come from advanced vehicle design and advances incombustion technology, but most of the reduction will come from advancedemission controls of which NO_(x) reduction catalysts are the centraltechnology. Similar reductions will be required of heavy diesel trucksin the near future, hence the need for new technologies having thecapability of achieving very high reduction of NO_(x) from lean burnengines, and at low operating temperatures as well (150° C.-250° C.).

Although the NO_(x) gases may be thermodynamically unstable with respectto decomposition into elemental oxygen and nitrogen, no simple,economical method or catalyst has been described for inducing thisdecomposition at high enough rates over broad temperature ranges to makelean NO_(x) reduction economically feasible. It has been discovered,however, that the addition of a reductant such as ammonia to the exhaustgas, under appropriate conditions, converts NO_(x) to elemental nitrogenand steam.

The process of contacting an industrial flue gas with a catalyst in thepresence of ammonia at a temperature usually in the range of about 200degrees Celsius to about 600 degrees Celsius to reduce NO_(x) in theflue gas is commonly known as the process for Selective CatalyticReduction (SCR) of NO_(x). Any reference made herein to “SelectiveCatalytic Reduction,” or to “SCR,” is intended to refer to a process inwhich a mixture of NO_(x) and NH₃ are induced to react catalytically inthe presence of oxygen at elevated temperatures.

For lean burn engine technology to be implemented, catalytic convertersmust be developed for lean burn engines. The catalyst employed in theconverter must be active over a broad range of temperature (usually inthe range of about 150-500 degrees Celsius, or broader is better), musthave very high activity for the conversion of NO_(x) to elementalnitrogen (N₂) and water (H₂O), must react with a broad range of NO andNO₂ in the gas sent from the engine to the catalytic converter, must besulfur tolerant, and should not produce N₂O or only a few ppm at most.With these goals in mind, lean burn catalysts that remove NO_(x) fromexhaust streams (i.e. deNO_(x) catalysts) are highly sought after andare the focus of considerable research worldwide.

A rather narrow window of satisfactory operating temperatures hascharacterized most catalysts for lean burn applications. Specifically,they only effectively convert NO_(x) over narrow temperature ranges thatdo not always match the temperatures at which the NO_(x) is emitted.Some of the better catalyst materials have included metal-substitutedzeolite catalysts such as Cu-ZSM-5, Fe-ZSM-5, and related catalystsconsisting of various zeolites with metal ions substituted into thezeolite structure. These materials are better in some ways thanconventional platinum-based deNO_(x) catalysts, but usually the bestoperating temperature ranges are too high (above 400 degrees Celsius)and too narrow (only about a hundred degrees Celsius in effectivetemperature width) for many practical applications. One significantadvantage that such ‘base metal’ (non-precious metal) zeolite catalystshave over Pt or other precious metal-containing catalysts (e.g. Pt, Pd,Rh, Ir) is that precious metal containing catalysts are known to producecopious quantities of N₂O in addition to N₂ under lean burn conditions.N₂O emissions are not yet regulated, but because N₂O is a potentgreenhouse gas, it is a very undesirable byproduct, and atechnologically useful catalyst should produce little, if any N₂O.

It has been recognized that the mechanism of SCR of NO_(x) in thepresence of ammonia requires an approximately 1:1 ratio of NO to NO₂ inthe feed stream to achieve the highest rates of NO_(x) reduction. Ratiosabove or below 1 result in significantly slower rates of NO_(x)reduction. As combustion processes generate NO_(x) mixtures having veryhigh NO/NO₂ ratios, i.e. the engine emits mostly NO, a process forreduction of the NO must include a method for oxidizing some of the NOto NO₂; preferably about half of the NO should be converted, resultingin a nearly 1:1 ratio.

Emissions control systems for mobile applications are likely to have anoxidation catalyst upstream from the NO_(x) reduction catalyst tooxidize unburned hydrocarbons. This oxidation catalyst will convert someof the NO to NO₂, perhaps up to 20 to 30 percent at a temperature ofabout 150 degrees Celsius, but not the 50 percent required to achievethe fastest rates of NO_(x) reduction. Because most NO_(x) catalysts arenot capable of oxidizing NO to NO₂ at low temperature, these catalystscannot assist the hydrocarbon oxidation catalyst to generate theadvantageous mixture of NO/NO₂ and so these catalysts are largelyineffective at low temperatures, that being below 300 degrees Celsiuswhere the feed contains mostly NO.

A strategy for improving the low temperature activity of SCR catalystsis to provide an additional non-precious metal containing catalyst thatcan oxidize NO to NO₂ so that the highest rates of NO_(x) reduction canbe realized. This is a strategy employed with the present invention.

In addition, internal combustion engines emit a large amount of unburnedhydrocarbons during cold engine start-up. In fact, a large fraction ofthe total emitted hydrocarbons released during the first minutes ofengine operation are due to the uncombusted hydrocarbons. Such releaseof hydrocarbons after cold engine start-up poses a special problem, asat that point the temperatures of the exhaust gas and the catalyticconverter are generally not high enough for conversion of the gaseouspollutants by conventional catalysts. The catalysts in present catalyticconverter systems are generally ineffective at ambient temperatures andmust reach high temperatures, often in the range of 300 degrees Celciusto 400 degrees Celcius, before they become effective. During this timeperiod, unburned hydrocarbons may adsorb onto the catalyst, causing afurther diminution in activity. Indeed, under some circumstances, theadsorbed hydrocarbons may form carbonaceous deposits, requiring hightemperatures to remove the deposit oxidatively. This can lead toirreversible damage of the catalyst. Therefore, catalysts that can avoidhydrocarbon deposition at low temperature, or more preferably, oxidizeunburned hydrocarbons at the lower temperatures, are highly desired.

SCR processes offer the possibility that unspent ammonia reductant couldbe emitted to the environment. As ammonia is a regulated toxicsubstance, there are stringent emissions standards for ammonia.Therefore, another desired feature for a broad temperature range SCRprocess is one in which very little, if any ammonia is allowed to escapeinto the atmosphere, even under strenuous transient conditions where theprocess temperature is increasing rapidly because of load on the engine.In other words, the catalytic NO_(x) reduction process should consumeall of the ammonia, or the catalyst should consume any excess ammonia byoxidation. In the latter case, it would be highly advantageous if theoxidation of any excess ammonia did not result in the formation of moreNO_(x), but that the oxidation process resulted in the net oxidation ofammonia to N₂— a so-called selective catalytic oxidation process.

A number of zeolite-based catalysts for SCR of NO_(x) with ammonia aredescribed below. Many of these catalysts where their activity is givenhave been tested in the forms of powders or compacted powders. In thesecatalytic tests, the flows through the catalyst beds are given in termsof gas hourly space velocity (GHSV). The GHSV is the volume of exhaustpassed in one hour divided by the volume of the catalyst bed, and isrelated to the residence time or reaction time that the gaseous specieshave to react on the catalyst before they leave the catalyst bed. It isgenerally desirable to minimize the catalyst volume to the extentpossible, and a useful catalyst should have high activity at high GHSV.For combustion processes, the GHSV is typically in a range from about20,000 h⁻¹ to about 200,000 h⁻¹. One difficulty in comparing theactivity of one catalyst to another when relative flow rates are givenin terms of GHSV arises when one tries to compare a compacted powdercatalyst with a catalyst that is supported on a monolith. In a powdercatalyst, the bed volume is measured in a straightforward manner. If thecatalyst is supported on a monolith such as a commercial cordieritehoneycomb support, then the catalyst volume is given as the volume ofthe honeycomb. The problem here is that the amount of catalyst supportedon the honeycomb is very small; most of the volume of the honeycombcatalyst is void space and the volume of the honeycomb itself. Thismakes it very difficult to make a simple comparison of catalyst activitybetween a powder catalyst and a monolith-supported catalyst. A rule ofthumb that is commonly used is to make a rough comparison in activitybetween a powder catalyst and a monolith catalyst is to multiply theGHSV of the powder catalyst by about 4, or conversely to divide the GHSVof the monolith catalyst test result by about 4. For example, if apowder catalyst is reported to have a certain activity at 30,000 h⁻¹GHSV, then it should be compared to a monolith catalyst at roughly 7,500h⁻¹ GHSV. Conversely, if a monolith catalyst has been reported to have acertain activity at 30,000 h⁻¹ GHSV, then the powder catalyst should becompared at a GHSV of about 120,000 h⁻¹.

The use of zeolite-based catalysts for the SCR of nitrogen oxides withammonia is well established. U.S. Pat. No. 4,220,632 to D. R. Pence etal. entitled “Reduction of Nitrogen Oxides With Catalytic Acid ResistantAluminosilicate Molecular Sieves and Ammonia,” incorporated by referenceherein, discloses the catalytic reduction of noxious nitrogen oxides ina waste stream (stack gas from a fossil-fuel-fired power generationplant or other industrial plant off-gas stream) using ammonia asreductant in the presence of a zeolite catalyst in the hydrogen orsodium form having pore openings of about 3 to 10 Angstroms.

U.S. Pat. No. 4,778,665 to Krishnamurthy et al. entitled “Abatement ofNO_(x) in Exhaust Gases,” incorporated by reference herein, describes anSCR process for pretreating industrial exhaust gases contaminated withNO_(x) in which the catalyst includes an intermediate pore zeolite witha silica to alumina ratio of at least 50 with a Constraint Index of 1 to12. These zeolites are sometimes referred to as ZSM-5 type zeolites. Thezeolite is preferably in the hydrogen form or has up to about 1 percentof a platinum group metal. According to the '665 patent, the hydrogenform of zeolite ZSM-5 (HZSM-5) catalyzes the SCR reaction attemperatures between about 400 degrees Celsius to about 500 degreesCelsius. At temperatures below about 400 degrees Celsius, HZSM-5 issignificantly less efficient at removing nitrogen oxides from the gasstream. These catalysts were tested as compacted powder extrudates atspace velocities below 10,000 h⁻¹.

U.S. Pat. No. 5,520,895 to S. B. Sharma et al. entitled “Method for theReduction of Nitrogen Oxides Using Iron Impregnated Zeolites,” whichissued on May 28, 1996 and is hereby incorporated by reference,describes a process employing impregnated zeolites as catalysts for theSCR of NO_(x) in exhaust gas. A catalyst used with this process includesan intermediate pore size zeolite powder that has been contacted with awater-soluble iron salt or salt precursor to produce an iron loading ofat least 0.4 weight percent, and a binder such as titania, zirconia, orsilica. The impregnated zeolite is calcined and hydrothermally treatedat a temperature of about 400-850 degrees Celsius to produce a catalystthat is capable of greater than 80 percent conversion of the NO_(x) toinnocuous compounds when the catalyst has been aged using 100 percentsteam at 700 degrees Celsius for 7 hours prior to sending the exhaustgas over the catalyst. These catalysts were tested as powders at spacevelocities of 12,000 h⁻¹.

U.S. Pat. No. 6,514,470 to K. C. Ott et al. entitled “Catalysts for LeanBurn Engine Exhaust Abatement,” hereby incorporated by reference,describes the catalytic reduction of nitrogen oxides in an exhauststream gas using a reductant material and an aluminum-silicate typecatalyst having a minor amount of a metal. Nitrogen oxides were reducedin the exhaust stream gas by at least 60 percent at temperatures withinthe range of from about 200 degrees Celcius to about 400 degreesCelcius. The term “hydrocarbons” as it is used both in the '470 patentand in the present patent application, is meant to refer to hydrocarbonsand also to partially oxidized products of hydrocarbons such asoxygenated hydrocarbons (alcohols, ketones, and the like). Whilehydrocarbons were used as the reductant in the examples disclosed in the'470 patent, it was mentioned that ammonia could also be used. However,no data using ammonia as reductant was presented.

In “A Superior Catalyst for Low-Temperature NO Reduction With NH₃,”Chem. Communications, (2003), pp. 848-849, incorporated by referenceherein, Gongshin Qi and Ralph T. Yang report an SCR process using aMn—Ce mixed-oxide powder catalyst that yields nearly 100 percent NOconversion at 100-150 degrees Celsius at a space velocity of 42,000 h⁻¹(as a powder), and that SO₂ and water have only slight effects on theactivity. The catalyst activity was shown to be dependent on therelative amount of Mn in the catalyst, and also on the calcinationtemperature used to prepare the catalyst.

In “Low-Temperature SCR of NO With NH₃ over USY-Supported ManganeseOxide-Based Catalysts,” Catalysis Letters, vol. 87, nos. 1-2, April2003, pp. 67-71, incorporated by reference herein, Gongshin Qi, Ralph T.Yang, and Ramsay Chang report the SCR of NO with ammonia and excessoxygen using manganese oxide, manganese-cerium oxide, and manganese-ironoxide supported on USY (ultrastable (i.e. high Si/Al) Y zeolite). It wasfound that MnO_(x)/USY had high activity and high selectivity tonitrogen at temperatures of from 80-180 degrees Celsius, and that theaddition of iron oxide or cerium oxide increased NO conversion. Acatalyst of 14% cerium and 6% manganese impregnated into ultrastable Yzeolite produced nearly 100 percent conversion of NO at 180 degreesCelsius at gas hourly space velocity (GHSV) of 30,000 h⁻¹ as a powdercatalyst. The only reported product was N₂ (with no N₂O) below 150degrees Celsius.

There remains a need for catalysts that show better performance for SCRof NO_(x), especially catalysts that reduce NO_(x) at temperatures lessthan about 200 degrees Celsius at high space velocities, particularlywhen supported on monolith honeycomb supports.

Therefore, an object of the present invention is to provide a catalystfor the selective catalytic reduction of NO_(x) in the presence ofammonia that shows excellent conversion at temperatures below 200degrees Celsius at space velocities greater than 30,000 h⁻¹ when testedas a monolith-supported catalyst, or 120,000 h⁻¹ when tested as apowder.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

SUMMARY OF THE INVENTION

In accordance with the purposes of the present invention, as embodiedand broadly described herein, the present invention includes a methodfor the Selective Catalytic Reduction of nitrogen oxides. The methodinvolves contacting an exhaust gas stream that includes nitrogen oxides,ammonia, and oxygen, with a catalyst under conditions effective tocatalytically reduce the nitrogen oxides such that less than about 0.6percent N₂O is generated. The catalyst is a medium pore zeolite that hasbeen ion exchanged with iron to provide an efficient SCR function andimpregnated with manganese and cerium to provide an efficient functionfor the low temperature oxidation of NO to NO₂ and any unburnedhydrocarbons. These invention catalysts that combine these dualfunctions are termed ‘hybrid’ catalysts, as they are hybrids of SCRcatalysts and potent NO oxidation catalysts.

The invention also includes a supported catalyst effective for theSelective Catalytic Reduction of nitrogen oxides in the presence ofammonia. The catalyst is supported on a monolith, and includes ZSM-5zeolite that has been exchanged with iron and impregnated with manganeseand cerium.

The invention also includes a method for catalytically reducing nitrogenoxides in an exhaust gas stream that contains nitrogen oxides, ammonia,and oxygen. The method involves contacting the exhaust gas stream underconditions effective to catalytically reduce the nitrogen oxides withFe-ZSM-5 catalyst and thereafter with a second catalyst. The secondcatalyst includes a medium pore zeolite ion that has been ion exchangedwith iron and impregnated with manganese and cerium.

The invention also includes a method for improving the low temperatureperformance of a lean NO_(x) trap. The method involves putting acatalyst upstream of a lean NO_(x) trap. The catalyst is a medium porezeolite impregnated with iron, cerium, and manganese. When the catalystis contacted with NO and oxygen, it substantially oxidizes the NO to NO₂to produce a NO₂ enriched stream that improves the low temperatureperformance of the lean NO_(x) trap.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiment(s) of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 shows a graphical representation of percent NO_(x) conversion asa function of temperature at a gas hourly space velocity (GHSV) of30,000 hr⁻¹ (monolith) and in a temperature range from 100 degreesCelsius to 400 degrees Celsius for the following catalysts and NO_(x)compositions: FeZSM-5/MnO_(x), 4:1 NO/NO₂ (filled circles);FeZSM-5/CeOx/MnO_(x), 4:1 NO/NO₂ (filled squares); powder catalystMnO_(x) supported on Y zeolite, 120,000 hr⁻¹ (hollow circles);Ce/MnO_(x) supported on Y zeolite, 4:1 NO/NO₂ (filled diamonds);FeZSM-5, 4:1 NO/NO₂ (filled triangles); and FeZSM-5, 1:1 NO/NO₂ (hollowdiamonds).

FIG. 2 shows a graphical representation of % (NO₂/(NO+NO₂)) as afunction of temperature in the temperature range of from about 100degrees Celsius to about 500 degrees Celsius for the following catalystsand conditions: monolith supported FeZSM-5/MnO_(x), 30,000 hr⁻¹ (hollowcircles); invention catalyst monolith-supported FeZSM-5/CeO_(x)/MnO_(y),30,000 hr⁻¹ (hollow squares); monolith supported Pt, space velocity of30K (filled diamonds); powder catalyst CeO_(x)/MnO_(x) on Y zeolite,120,000 hr⁻¹ (filled squares); powder catalyst MnO_(x) supported on Yzeolite 120,000 hr⁻¹ (filled triangles); and powder catalyst MnZSM-5,30K GHSV (x-shaped symbols). The uppermost curve for temperatures up toabout 350 degrees Celsius is the equilibrium curve for the oxidation ofNO with oxygen to produce NO₂ (hollow circles). FIG. 2 shows that theinvention catalysts are superior catalysts for the oxidation of NO toNO₂ at low temperatures, which is a requirement for achieving high ratesof NO_(x) conversion by SCR in the presence of ammonia.

FIG. 3 shows a graphical representation of % NOx conversion versustemperature in the temperature range of from about 100 degrees Celsiusto about 400 degrees Celsius for the following catalysts and conditions:FeZSM-5 (hollow circles); CeOx/FeZSM-5 after the first impregnation withCe (hollow squares); CeOx/FeZSM-5 after the second impregnation with Ce(filled squares); and MnOx/CeOx/FeZSM-5 (hollow triangles). Ceriumimpregnation was accomplished by immersing iron zeolite into an aqueoussolution of cerous nitrate, followed by drying and calcination toconvert the cerous nitrate into cerium oxides (CeO_(x)). As FIG. 3shows, the measured activity of the catalyst before and afterimpregnation were not significantly different. However, the addition ofMnO_(x) in combination with the cerium oxides improved the performanceof the catalyst at lower temperatures (below about 250 degrees Celsius).

FIG. 4 a and FIG. 4 b show graphs related to results of NO_(x)conversion and NO conversion for catalysts A, B, C, D, and E prepared bydip coating a monolith with catalyst solution, drying, and calcination.Catalyst A was prepared by dip-coating a monolith into solutions of 2molar cerous nitrate and 2 molar manganous nitrate. Catalyst B wasprepared by dip-coating a monolith into solutions of 1 molar cerousnitrate and 1 molar manganous nitrate. Catalyst C was prepared bydip-coating a monolith into solutions of 0.5 molar cerous nitrate and0.5 molar manganous nitrate. Catalyst D was prepared by dip-coating amonolith into solutions of 1 molar manganous nitrate and 0.5 molarcerous nitrate. Catalyst E was prepared by dip-coating a monolith intosolutions of 0.5 molar manganous nitrate and 1 molar cerous nitrate.Catalyst A was re-impregnated with the aqueous 2 molar solution ofmanganese nitrate and cerium nitrate, dried, and calcined to generateCatalyst F. Catalyst F was impregnated again in the cerium and manganesesolution, dried, and calcined to generate Catalyst G. The results ofNO_(x) conversion and NO conversion for catalyst F and for catalyst Gare also shown in FIG. 4 a and FIG. 4 b.

FIG. 5 shows results related to the stability of the Ce, MnO_(x)Fe-ZSM-5/monolith catalyst of the present invention to aging usingsulfur trioxide (SO₃).

FIG. 6 shows the results of NOx conversion as a function of temperaturefor a dual bed catalyst of the present invention.

DETAILED DESCRIPTION

The invention relates to the Selective Catalytic Reduction (SCR) ofNO_(x) in the presence of ammonia and excess oxygen over a broadtemperature range. The invention includes a catalyst that has been shownto convert gaseous mixtures of NO and NO₂ to N₂. The catalyst, anembodiment of which includes a monolith-supported medium pore zeoliteion-exchanged with small amounts of iron, manganese, and cerium, hasbeen demonstrated as being a highly active catalyst for the conversionof NO_(x) in the presence of ammonia in the temperature range from about200 degrees Celsius to about 400 degrees Celsius, and shows surprisingactivity at temperatures below 200 degrees Celsius, which is highlydesirable because state-of-the-art combustion engines are becoming soefficient that exhaust gas temperatures are dropping into the rangebelow 200 degrees Celsius. Activity at these low temperatures is alsodesirable because of the emission during ‘cold start’ conditions, whichoccurs before conventional catalysts become hot enough to displayeffective catalytic activity.

A standard by which deNO_(x) catalysts are currently being measured isthat they perform better than the best commercial catalyst if they canprovide better than 60 percent conversion at a temperature of 200degrees Celsius when 20 percent of the feed into the catalyst isnitrogen dioxide (NO₂). The catalyst of the invention has been shown tomeet this standard by displaying greater than 60 percent conversion of agaseous, 4 to 1 mixture of NO/NO₂ at 200 degrees Celsius.

The exhaust gases that can be treated in the catalytic system of thepresent invention can come from the combustion of fuels in automotiveengines (such as diesel engines), gas turbines, engines using anoxygen-rich mixture (lean-burn conditions), and electrical powergeneration stations. The term “exhaust gas” means any waste gas that isformed in an industrial process or operation and that is normallydisposed of by discharge to the atmosphere, with our without additionaltreatment. “Exhaust gas” includes the gas produced by internalcombustion engines. The composition of such a gas varies and depends onthe particular process or operation that leads to its formation. Whenformed in the combustion of fossil fuels, it will generally includenitrogen, steam and carbon dioxide. In addition it will also containsmaller amounts of NO_(x). The fuels can be, for example, natural gas,gasoline, LPG, kerosene, heavy oil and coal. Fuels that contain sulfurwill typically produce an exhaust gas that contains one or more sulfuroxides. Rich fuel-air mixtures will generally produce an exhaust gasthat contains little if any free oxygen along with some carbon monoxide,hydrocarbons (where the term ‘hydrocarbons’ is meant to include bothhydrocarbons and partially oxidized hydrocarbons, as described earlier),and hydrogen. Lean fuel-air mixtures, which contain more air than whatis stoichiometrically required to completely burn the fuel, will form anexhaust gas that contains oxygen. Other industrial processes such asnitration, uranium recovery, and calcining solid salts that containnitrates produce exhaust gases that can have compositions different fromthose produced from combustion of fossil fuels. They may besubstantially devoid of steam, for example, and may contain very highconcentrations of nitrogen or other materials.

The conversion of NO₂ and NO to nitrogen in the presence of ammonia isbelieved to process generally according to equations (1) and (2) and (3)below.2NO₂+4NH₃+O₂→3N₂+6H₂O  (1)4NO+4NH₃+O₂→4N₂+6H₂O  (2)2NO+2NO₂+4NH₃→4N₂+6H₂O  (3)

The highest rates of NO_(x) conversion by reduction with ammonia occurwhen the ratio of NO to NO₂ is 1 (equation 3). To achieve the high ratedue to the ‘fast’ SCR process of equation 3 from an exhaust stream thatcontains little NO₂ and mostly NO as the NO_(x) component, sufficient NOmust be first oxidized to NO₂ to achieve a ratio of NO to NO₂ that isclose to 1.

The present invention is effective for treating exhaust gas containingthe approximate stoichiometric amount of ammonia. The ammonia may bepresent in the gas, may be added to the gas, or may be produced by anupstream process (such as by decomposition of urea). The term“approximate stoichiometric amount” means from about 0.75 to about 1.25times the molar amount of ammonia indicated in equations (1), (2), and(3) above.

In performance testing of the catalysts and processes of the presentinvention, a simulated exhaust gas mixture was used. Those skilled inthe art will readily recognize that other variations of such simulatedexhaust gas mixtures could be employed.

The exhaust gas is typically treated in the catalytic system of thisinvention at a temperature of from about 50 degrees Celsius to about1000 degrees Celsius or more, e.g. within the range of within the rangeof about 100 degrees Celsius to about 900 degrees Celsius, e.g. of about100 degrees Celsius to about 600 degrees Celsius, e.g. of about 120degrees Celsius to about 600 degrees Celsius, e.g. of about 120 degreesCelsius to about 400 degrees Celsius, and at a gas hourly space velocity(GHSV, relating to volumes of gas at standard temperature and pressure(STP) per volume of catalyst per hour) adjusted to provide the desiredconversion. The GHSV can be from about 1,000 to about 500,000 hr⁻¹, e.g.within the range of about 2,500 to about 250,000 hr⁻¹, e.g. from about5,000 to about 150,000 hr⁻¹, e.g. from about 10,000 to about 100,000hr⁻¹.

The process of the invention is operable at subatmospheric tosuperatmospheric pressure, e.g. from about 5 psia to about 500 psia,preferably from about 10 psia to about 50 psia, i.e. near or slightlyabove atmospheric pressure.

The gas mixture directed over the catalyst should contain at least astoichiometric amount of oxygen as indicated by equations (1) and (2),or enough oxygen to convert half the NO to NO₂ to proceed by the way ofthe ‘fast’ SCR reaction (equation 3). Excess levels of oxygen above thestoichiometric amount may be desirable. According to the method of theinvention, a source of oxygen, such as air, is sent to the catalystalong with the exhaust gas. If sufficient oxygen is not present in theexhaust gas, a source of oxygen, e.g. air, may be added to the exhaustgas, and if sufficient oxygen is present in the exhaust gas, then no airneed be added to the exhaust gas.

Adequate conversion may be readily achieved with a simple stationaryfixed-bed of catalyst. The fixed includes a bed of the inventioncatalyst, which is a ZSM-5 type zeolite having small amounts of iron,manganese, and cerium impregnated therein. This bed can be used alone,or in combination with other catalyst beds, e.g. a dual bed. A bed ofthe invention catalyst can be used in combination with another catalystbed such as a FeZSM-5 catalyst bed, to provide a dual bed that may becapable of even better overall performance than a single bed over thewide range of operating conditions for a combustion engine, for example.

Suitable mixing may be used before the gas reaches the catalyst toproduce a homogeneous gas mixture for catalytic conversion. The mixersmay be any suitable arrangement, including, for example, baffles, discs,ceramic discs, static mixers, or combinations of these. The mixing maybe integral with the gas flow paths.

Catalysts useful with the present invention typically include an activematerial and a support. Suitable support materials include cordierite,nitrides, carbides, borides, intermetallics, mullite, alumina, naturaland synthetic zeolites, lithium aluminosilicate, titania, feldspars,quartz, fused or amorphous silica, clays, aluminates, zirconia, spinels,or metal monoliths of aluminum-containing ferrite type stainless steel,or austenite type stainless steel, and combinations thereof.

Typical substrates are disclosed in U.S. Pat. Nos. 4,127,691 and3,885,977, incorporated by reference herein. The catalyst is combinedwith the substrate in any method that ensures that the catalyst willremain intact during the catalytic reaction.

A catalyst useful with the present invention comprises a medium porezeolite, whether naturally occurring or synthesized crystallinezeolites. Preferably these zeolites are medium pore zeolites with asilica to alumina ratio of at least 50. Examples include ZSM-5, ZSM-11,ZSM-12, ZSM-21, ZSM-23, ZSM-35, ZSM-38, and ZSM-48. Preferably, thezeolite is ZSM-5.

An embodiment of the catalyst of the present invention was prepared byion exchanging a zeolite with iron and then impregnating manganese andcerium onto the iron zeolite in a stepwise fashion. The products weretested at various stages during the preparation. First, a slurry ofZSM-5 was coated onto a monolith of cordierite monolith (the support).The slurry-coated monolith was then dried at a temperature of about450-500 degrees Celsius. Following the drying, ferric chloride (FeCl₃)was sublimed onto ZSM-5 treated cordierite, after which the product wassubjected to calcination at a temperature of about 500 degrees Celsius.This material is abbreviated “FeZSM-5” on FIG. 1 and FIG. 2. Theactivity of this product was determined by performing SelectiveCatalytic Reduction (SCR) of NO_(x) using two different gaseous mixturesof nitrogen oxides, 1:1 mixture of NO/NO₂ and a 4:1 mixture of NO/NO₂,in the presence of about 1 equivalent of ammonia and excess oxygen gas.The conversion data for both the 1:1 mixture and the 4:1 mixture areeach reported in FIG. 1, which shows a graphical representation ofpercent NO_(x) conversion as a function of temperature at a GHSV of30,000 hr⁻¹ and in a temperature range from 100 degrees Celsius to 400degrees Celsius. The curve with hollow diamond symbols shows theconversion data for the 1:1 mixture, and the curve with triangle symbolsshows the conversion data for the 4:1 mixture. As FIG. 1 shows, thematerial prepared by subliming FeCl₃ onto ZSM-5 converts about 65percent of the NO_(x) from a feed stream of 4:1 NO/NO₂ at 200 degreesCelcius.

Cerium was added to the iron zeolite catalyst by immersing the catalystinto an aqueous solution of cerous nitrate. The drying and calciningsteps converted the cerous nitrate into cerium oxides (CeO_(x)). Theactivity of this catalyst was measured, and was found not to besignificantly different than the starting iron catalyst. The data areplotted on the graph shown in FIG. 3.

The iron- and cerium-containing product catalyst prepared as describedabove was immersed into an aqueous solution of manganese (II) nitrate,then dried and calcined. The drying and calcinations step are believedto convert at least some of the manganese nitrate into manganese oxides(MnO_(x)) and possibly mixed cerium-manganese oxides. The activity ofthis catalyst was determined by performing Selective Catalytic Reduction(SCR) with a gas mixture containing a 4:1 mixture of NO/NO₂, about oneequivalent ammonia, and excess oxygen gas. The conversion data are shownin FIG. 1 (filled circles). As FIG. 3 shows, the addition of MnO_(x) incombination with the cerium oxides improved the performance of thecatalyst by acting as a potent oxidizer, as demonstrated by improvedconversion of 4:1 mixture of NO/NO₂ at the lower temperatures.

The performance of the invention catalyst was compared to that of acatalyst powder described by Gonshin Qi et al. in the following paper:“Low-Temperature SCR of NO with NH₃ over USY-supported ManganeseOxide-Based Catalysts,” Catalysis Letters, vol. 87, nos. 1-2. The Qi etal. powder catalyst is composed of 14 percent cerium and 6 percentmanganese impregnated into ultrastable (i.e. high Si/Al) Y zeolite,which was reported to promote nearly 100 percent conversion of NO at 180degrees Celsius under the disclosed experimental conditions. This powdercatalyst produced 80 percent conversion when wet at a temperature of 150degrees Celsius with a feed of 100% NO and a space velocity of 30,000GHSV. It produced 55 percent conversion at a space velocity of 120,000GHSV, a temperature of 150 degrees Celsius, and a gas composition of 4:1NO/NO₂. This is an interesting catalyst and appears to be oxidizing alarge fraction of NO to NO₂ to achieve these rates. For NO oxidation at120,000 GHSV, the results are very similar to that for Platinum. As FIG.1 and FIG. 2 show, the CeO_(x)/MnO_(x)/Y catalyst of Qi, Yang, and Changhas even better low temperature performance than FeZSM-5. However, asFIG. 1 and FIG. 2 show, the catalyst of the present invention,Ce/MnOx/FeZSM-5 has substantially better low temperature performance.

Another aspect of the present invention relates to “dual bed” catalystsystems. As these new catalysts are too active for ammonia oxidation athigh temperature but have fantastic low temperature performance, onesolution to get a broader range of operation involves putting a “hightemperature catalyst” such as FeZSM-5 that has excellent performance athigh temperature in front of a bed of the hybrid catalysts of theinvention. At high temperatures, the high temperature catalyst convertsNO_(x) to nitrogen with high efficiency. At low temperature, however,the “high temperature” catalyst is capable of only converting around 60percent of the NO_(x) at 200 degrees Celsius at 4:1 NO/NO₂ feeds. Thehybrid catalyst would efficiently convert a large fraction of theremaining NO_(x), likely attaining better than 90 percent conversionover a broad temperature range of about 150 degrees Celsius. This dualfunctioning catalyst bed enables NO_(x) conversion over broadtemperature ranges from 150 degrees Celsius to greater than 450 degreesCelsius, even up to 500 degrees Celsius.

The following EXAMPLES illustrate the operability of the invention.

EXAMPLE 1

Preparation of Fe ZSM-5 supported on cordierite monolith. Cylinders ofcordierite monolith (10 mm in diameter×12 mm long; 400 cells per inch²)were coated with H-ZSM-5 (ZEOLYST INTERNATIONAL) zeolite by dipping themonolith into an aqueous slurry of the zeolite powder followed by dryingat 110 degrees Celsius. Several cycles of coating followed by dryingwere necessary to achieve a zeolite coating of around 20 to 24 percentby weight. The coated monolith was then calcined at a temperature ofabout 500 degrees Celsius. Iron was exchanged into the pores of thezeolites by the well-known method of gas-phase exchange using FeCl₃ asthe volatile iron component. A piece of ZSM-5 coated cordierite monolithwas placed into a boat downstream from a boat of anhydrous FeCl₃. Theboats were contained within a quartz apparatus that was purged with drynitrogen gas. The quartz apparatus was heated to a temperature of frombetween 300 degrees Celsius and 325 degrees Celsius to initiate thesublimation of FeCl₃. After the zeolite-coated monolith had changed to acolor of yellow to yellow orange, the apparatus was cooled under drynitrogen, and the resulting monolith was calcined in ambient air at atemperature of about 500 degrees Celsius to yield a catalyst referred toas Fe-ZSM-5/monolith. The catalyst gained approximately 7 percent byweight of FeO_(x) based on the weight of the zeolite coating.

EXAMPLE 2

Testing of Fe ZSM-5 monolith catalyst. The Fe-ZSM-5/monolith preparedaccording to EXAMPLE 1 was tested for NO_(x) conversion activity and forNO oxidation activity. To measure the activity for NO_(x) conversion,the Fe-ZSM-5/monolith was placed into a 10 mm diameter quartz reactortube. Reaction gases (NO, NO₂, NH₃, and O₂ in He) were blended usingmass flow controllers to produce a gas mixture having 350 parts permillion (ppm) total NO_(x), 350 ppm NH₃, and 12 percent O₂; 5 percentsteam was added using a syringe pump and an evaporator. The NO:NO₂ ratiowas either 1:1 or 4:1. The space velocity was either 30,000 h⁻¹ or60,000 h⁻¹. Products and reactants (NO, NO₂, N₂O, NH₃) were analyzedusing a Fourier Transform Infrared (FT-IR) spectrometer with a heatedcell having a 2 meter, or 10 meter, path length. Nitrogen was measuredusing a gas chromatograph. Operation of the reactor was automated.Catalytic performance data was obtained over the temperature range of500 degrees Celsius to 120 degrees Celsius. The data is summarized inFIG. 1.

To determine the NO oxidation activity, a blend of 190 ppm NO and 12percent O₂ in He was fed to the reactor, and the quantity of NO₂ tototal NO_(x) was measured using FT-IR. The results of the NO oxidationtest are shown in FIG. 2.

EXAMPLE 3

Preparation of MnO_(x)/Y catalyst. MnOx supported on zeolite Y catalystcomparable to a catalyst reported by Yang et al. (“A Superior Catalystfor Low-Temperature NO Reduction With NH₃,” Chem. Commun. (2003), pp.848-849) was prepared by incipient wetness impregnation. A six gramsample of zeolite Y (ZEOLYST®) was impregnated with approximately 3cubic centimeters (cc) of 5 molar manganous nitrate solution to achievea catalyst having approximately 10 to 15 percent by weight Mn. Thecatalyst was dried at a temperature of about 120 degrees Celsius, andthen calcined at a temperature of about 500 degrees Celsius.

EXAMPLE 4

Testing of MnO_(x)/Y catalyst. The MnO_(x)/Y catalyst prepared inEXAMPLE 3 was tested for NO_(x) conversion in an identical fashion asgiven in EXAMPLE 2. This catalyst was tested as a powder diluted in 1.5cc of crushed cordierite. The GHSV of 120,000 h⁻¹ (based on volume ofactive catalyst powder) was chosen to make a good comparison to othercatalytic results from monolith catalysts. The results for the 4:1NO/NO₂ conditions are shown in FIG. 1. The NO oxidation activity of thiscatalyst at the same GHSV as the NO_(x) conversion experiment and as afunction of temperature was determined as described in EXAMPLE 2. Thedata are presented in FIG. 2.

EXAMPLE 5

Preparation and testing of MnO_(x)/Y catalyst. A Ce, MnO_(x) supportedon zeolite Y catalyst comparable to the catalyst reported by Yang et al(“A Superior Catalyst for Low-Temperature NO Reduction With NH₃,” Chem.Commun. (2003), pp. 848-849) was prepared by incipient wetnessimpregnation of a powdered sample of zeolite Y with an aqueous solutionof Ce and Mn nitrates to achieve a 6% Mn, 14% Ce by weight catalyst.After the incipient wetness impregnation was performed, the catalyst wasdried overnight at a temperature of about 120 degrees Celsius, and thencalcined at a temperature of about 500 degrees Celsius. The productCe,MnO_(x)/Y catalyst was tested for NO_(x) conversion as described inEXAMPLE 2. Ce,MnO_(x)/Y catalyst powder was diluted 50/50 with 1.5 cc ofcrushed cordierite. The GHSV of 120,000 h⁻¹ (based on volume of activecatalyst powder) was chosen to make a good comparison to other catalyticresults from monolith catalysts. The results for the 4:1 NO/NO₂conditions are shown in FIG. 1.

The NO oxidation activity of this catalyst at the same GHSV as theNO_(x) conversion experiment and as a function of temperature wasdetermined as described in EXAMPLE 2. The data is presented in FIG. 2.

EXAMPLE 6

Preparation of CeO_(x)—Fe-ZSM-5/monolith catalyst. The Fe-ZSM-5/monolithcatalyst from EXAMPLE 1 was dip coated in an aqueous 4M cerous nitratesolution. The excess solution was shaken off, and the catalyst dried ata temperature of about 120 degrees Celsius. The catalyst was thencalcined at a temperature of about 500 degrees Celsius. The product wasCeO_(x)—Fe-ZSM-5/monolith catalyst.

EXAMPLE 7

Testing of CeO_(x)—Fe-ZSM-5/monolith catalyst TheCeO_(x)—Fe-ZSM-5/monolith catalyst was tested for NO_(x) conversion inan identical fashion as described in EXAMPLE 2.CeO_(x)—Fe-ZSM-5/monolith catalyst powder was diluted 50/50 with 1.5 ccof crushed cordierite. The GHSV of 120,000 h⁻¹ was chosen to make a goodcomparison to other catalytic results from monolith catalysts. Theresults for the 4:1 NO/NO₂ conditions are shown in FIG. 3.

EXAMPLE 8

Preparation of CeO_(x)—Fe-ZSM-5/monolith catalyst. TheCeO_(x)—Fe-ZSM-5/monolith catalyst from EXAMPLE 6 was impregnated asecond time by dip coating in an aqueous solution of 4M cerous nitrate.The catalyst was dried and calcined as described in EXAMPLE 6.

EXAMPLE 9

Testing of CeO_(x)—Fe-ZSM-5/monolith catalyst. TheCeO_(x)—Fe-ZSM-5/monolith catalyst of EXAMPLE 8 (having the additionalimpregnation/calcination treatment) was tested for NO_(x) conversionusing the identical conditions as in EXAMPLE 2. The test result datashowed little difference from the data for the catalyst given in EXAMPLE7. The data are plotted in FIG. 3.

EXAMPLE 10

Preparation of Mn,CeO_(x)—Fe-ZSM-5 monolith catalyst. TheCeO_(x)—Fe-ZSM-5/monolith catalyst from EXAMPLE 7 was impregnated withMn by dip coating the monolith into an aqueous solution of 1 molarmanganous nitrate. The excess solution was shaken off and the catalystwas then dried at a temperature of about 120 degrees Celsius andafterward calcined at a temperature of about 500 degrees Celsius. Theproduct was the catalyst Mn,CeO_(x)—Fe-ZSM-5.

EXAMPLE 11

Testing of Mn, CeO_(x)—Fe-ZSM-5 monolith catalyst. TheMn,CeO_(x)—Fe-ZSM-5/monolith catalyst prepared as described in EXAMPLE10 was tested for NO_(x) conversion using the same conditions as inEXAMPLE 2. These data are shown in FIG. 1.

The NO oxidation capability of this catalyst was tested using 190 ppm NOas described in EXAMPLE 2. The data are shown in FIG. 3.

EXAMPLE 12

Preparation of five Mn,CeO_(x)—Fe-ZSM-5 monolith catalysts. A series ofcatalysts were prepared to examine the relationship of the amount of Ceand Mn and the effect on catalytic activity for NO_(x) conversion and NOoxidation. Five pieces of cordierite monolith (10 mm diameter, 12 mmlong, 400 cells per inch) were coated with an aqueous slurry of NH₄⁺-ZSM-5. The average zeolite loading was 34 percent by weight. Thecatalysts were dried, calcined, and treated with FeCl₃ as in EXAMPLE 1.These five identically prepared Fe-ZSM-5/monolith catalysts were thenimpregnated with varying amounts of Ce and Mn by dip-coating them intoaqueous solutions of cerium nitrate and manganese nitrate as follows:

Catalyst A was prepared by dip-coating a monolith into solutions of 2molar cerous nitrate and 2 molar manganous nitrate.

Catalyst B was prepared by dip-coating a monolith into solutions of 1molar cerous nitrate and 1 molar manganous nitrate.

Catalyst C was prepared by dip-coating a monolith into solutions of 0.5molar cerous nitrate and 0.5 molar manganous nitrate.

Catalyst D was prepared by dip-coating a monolith into solutions of 1molar manganous nitrate and 0.5 molar cerous nitrate.

Catalyst E was prepared by dip-coating a monolith into solutions of 0.5molar manganous nitrate and 1 molar cerous nitrate.

After dip-coating each monolith, the excess solution was removed fromeach of the monolith pieces, and all five pieces of catalyst were driedand calcined as described before.

EXAMPLE 13

Testing of five Mn, CeO_(x)—Fe-ZSM-5 monolith catalysts. The catalystsA-E from EXAMPLE 12 were tested for NO_(x) conversion and NO oxidationusing the method given in EXAMPLE 2. The results of NO_(x) conversionand NO conversion are given in FIG. 4 a and FIG. 4 b.

EXAMPLE 14

Preparation of two Mn, CeO_(x)—Fe-ZSM-5 monolith catalysts. Catalyst Afrom EXAMPLE 12 was re-impregnated with the aqueous 2 molar solution ofmanganese nitrate and cerium nitrate, dried, and calcined to generateCatalyst F. The NO_(x) conversion and NO conversion for catalyst F weremeasured. Afterward, catalyst F was impregnated again in the cerium andmanganese solution, dried, and calcined to generate Catalyst G. Theresults of NO_(x) conversion and NO conversion for catalyst F and forcatalyst G are shown in FIG. 4 a and FIG. 4 b.

EXAMPLE 15

Oxidation of ammonia using Mn, CeO_(x)—Fe-ZSM-5 monolith catalyst.Catalyst F was used to demonstrate the capability of usingMn,CeO_(x)—Fe-ZSM-5 monolith catalyst to oxidize ammonia and therebyminimize the loss of any excess or unconsumed ammonia (sometimes knownas the ammonia ‘slip’) into the environment. Catalyst F was placed intoa 10 mm diameter quartz reactor. A gas mixture including about 500 ppmof NH₃ and 12 percent O₂ diluted in He were delivered to the reactoralong with 5 percent steam. The conversion of ammonia was monitored byFT-IR and gas chromatography (GC) to detect N₂. At a temperature ofabout 300 degrees Celsius, the ammonia was completely converted. Theselectivity to N₂ was about 80 percent, and the selectivity to NO_(x)was about 20 percent.

EXAMPLE 16

Resistance of Mn, CeO_(x)—Fe-ZSM-5 monolith catalyst to hydrocarbonpoisoning. Catalyst F was used to demonstrate the resistance ofMn,CeO_(x)—Fe-ZSM-5 monolith catalyst to hydrocarbon poisoning. Whilemaintaining the catalyst at a temperature of 177 degrees Celsius, aNO/NO₂ ratio of 4:1, and the conditions given in EXAMPLE 2, the NO_(x)conversion of 95 percent was measured. A train of 4 pulses of 10microliters of liquid toluene injected at 4 minute intervals were thenvaporized and delivered upstream of the catalyst bed. After each pulse,a sharp decrement in NO_(x) conversion was noted, declining by about 10points, but then rapidly recovering to above 90 percent. After the 4pulses of toluene, NO_(x) conversion was greater than 90 percent, andrecovered to 95 percent conversion in less than 3 hours.

EXAMPLE 17

Stability of Ce, MNO_(x) Fe-ZSM-5/monolith catalyst to aging with SO₃.The Ce, MnO_(x) Fe-ZSM-5/monolith catalyst from EXAMPLE 10 was testedfor stability to SO₃ aging. A blend of 45 ppm SO_(x), mostly SO₃, in airwith steam was passed over the catalyst for 15 hours while thetemperature of the catalyst was held at a temperature of about 350degrees Celsius. The catalyst was then heated briefly to a temperatureof about 500 degrees Celsius, and the NO_(x) conversion in a 4:1 NO/NO₂blend was tested as outlined in EXAMPLE 2. The data are showngraphically in the plot shown in FIG. 5. The shape of the curve forNO_(x) conversion changed, but NO_(x) conversion remained greater than80 percent at a temperature of about 200 degrees Celsius, and wasgreater than 90 percent from a temperature of about 275 degrees Celsiusto a temperature of about 475 degrees Celsius.

EXAMPLE 18

Dual bed catalyst. To provide a broad temperature window process, dualbed catalysts were prepared. Catalyst F from EXAMPLES 14, 15, and 16 wasplaced downstream from a 10 mm diameter×12 mm long piece ofFe-ZSM-5/monolith catalyst prepared as described in EXAMPLE 1. The dualbed catalyst was tested as described in EXAMPLE 2, except that the flowwas doubled to account for the doubling of volume of the overall bed;thus the GHSV for the dual bed was 30,000 h⁻¹. The NO_(x) conversion asa function of temperature is shown in FIG. 6.

EXAMPLE 19

Dual bed catalyst. A dual bed catalyst was prepared by placing a 10 mmdiameter×5.4 mm diameter piece of Fe-ZSM-5 monolith catalyst prepared asin EXAMPLE 1 upstream from catalyst F (from EXAMPLE 18). The flow ratewas adjusted to give an overall GHSV of 30,000 h⁻¹. The results of theNO_(x) conversion test are shown in FIG. 6.

EXAMPLE 20

Stability of Dual Bed Catalyst to Hydrocarbon Poisoning. The dual bedcatalyst from EXAMPLE 19 was subjected to the standard 4:1 NO/NO₂ ratioNO_(x) conversion conditions at a temperature of 160° C. NO_(x)conversion was constant at 85% prior to introduction of toluene. Toluenevapor was introduced at a concentration of 100 ppm. During the next 2hours, the conversion of NO_(x) did not change. The concentration oftoluene was then increased to 1000 ppm. Over the next two hours, theNO_(x) conversion slowly dropped to about 65 percent. Upon heating thecatalyst bed to a temperature of about 250 degrees Celsius with 1000 ppmtoluene still in the feed, NO_(x) conversion rose to about 85 percentand was steady. All catalytic activity was regained when toluene wasremoved from the feed and the catalyst bed was heated briefly to atemperature of 250-300° C.

Lean NO_(x) traps are approach for the conversion of NO_(x) from leanburn engines. These traps operate by adsorbing NO₂ with an adsorbent,followed by reacting the adsorbed NO₂ to N₂. While this approachprovides yet another way to abate NO_(x) at higher temperatures, it isnecessary to convert NO to NO₂ in order for the trap to be effective.This conversion of NO to NO₂ is difficult at the lower temperatures ofthe desired operating range, particularly at temperatures from about 100degrees Celsius to about 250 degrees Celsius. One aspect of the presentinvention is related to a potent catalyst for the catalytic oxidation ofNO to NO₂ to enable a lean NO_(x) trap to operate more efficiently overa broader temperature range, particularly at temperatures below 250degrees Celsius. The ability of invention catalysts having Mn and Ce tooxidize NO to NO₂ at low temperatures allows lean NO_(x) traps tooperate more effectively at lower temperature by increasing the amountof NO₂ in the gas, and thus increasing the amount of NO_(x) trapped asNO₂. The invention catalyst having manganese and cerium is, therefore, alow temperature catalyst that, when placed upstream from a lean NO_(x)trap catalyst, improves the low temperature efficiency of the trapdevice.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching. For example, it is believed that while iron wasincorporated into the zeolite structure by sublimation, iron may also beincorporated by ion exchange techniques as well.

The embodiment(s) were chosen and described in order to best explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A method for improving the low temperature performance of a leanNO_(x) trap comprising putting a catalyst upstream of a lean NO_(x)trap, the catalyst comprising a medium pore zeolite impregnated withiron, cerium, and manganese, and then contacting the catalyst with NOand oxygen, whereby the catalyst substantially oxidizes the NO to NO₂ toproduce a NO₂ enriched stream that improves the low temperatureperformance of the lean NO_(x) trap.